Recently, evaporators have been adopted as an alternate water treatment system in oil recovery facilities, such as, for example, to treat produced water from a Steam Assisted Gravity Drainage (SAGD) facility or water discharge from a Hydraulic Fracture Oil Recovery Process (fracking). The control system enables the control of contaminants in the water which contaminants are introduced through and/or are specific to the particular oil recovery process in question. The contaminant control schemes developed are also applicable to Mechanical Vapor Compression (MVC) evaporator systems employed in other applications, especially those with a potential hydrocarbon or compressor fouling problem.
Water is used in many industrial processes for a variety of applications such as steam production, cooling, washing, diluting, scrubbing, and the like. In oil recovery processes, increased efforts have been made to conserve water by maximizing the reuse of process water and hence reducing the amount of waste water which is being discharged and thereby also minimizing the amount of fresh water introduced as make-up water. This result is both economical and has environmental benefits. However, reusing process water has its own challenges since generally the process water is contaminated in its initial use and requires additional treatment such as filtration, sedimentation, flocculation, evaporation, or chemical treatment before it can be reused. Treatment of the process water for reuse must in itself be efficient and economical before it will occur, with the extent of such treatment being determined by the intended use of the water.
One known treatment method is termed Mechanical Vapor Compression (MVC) evaporation. In this known method, a compressor is utilized to produce the pressure and temperature differential necessary to drive a known falling film exchanger to produce a high purity distilled water product and a concentrated brine product. One example of such a known system is illustrated in FIG. 1 herein.
Evaporators have been used extensively in the mining industry as well as in the pulp and paper industry in order to concentrate solids into a brine and/or to recover water from waste streams. In these applications, the solid contaminants are generally soluble in water. However, in oil recovery processes that is not the case. For example, in the SAGD process, as a result of injecting steam into an underground reservoir that is recovered as hot water with the production fluids, contaminants will be introduced into the water in various concentrations. Oil and water soluble solids present in the reservoir may cause variances in the quality of the produced water at any given time. This can lead to operating problems in standard evaporator designs.
In the SAGD industry, the produced water recovered from the SAGD production fluids, and make-up water added to account for losses, must be treated to remove various contaminants in order to meet boiler feed water specifications. The contaminants can include water hardness, silica, minerals, and residual oil/bitumen. If the water hardness, silica, and minerals are not removed from the water prior to steam generation via a boiler, they will precipitate in the boiler causing reduced heat transfer, lower capacities, higher boiler tube temperatures, and extended boiler outages, as the boiler needs to be cleaned and repaired and, ultimately, failure of the boiler. If the residual oil/bitumen is not removed from the brine in the evaporator sump, there will be foaming and fouling issues in the evaporator exchanger and sump, again leading to process upsets and shutdowns of the system.
The typical problems in evaporators in SAGD facilities include hardness scaling; silica deposits; oil accumulation and foaming; poor internal mist elimination performance; compressor vibration and scaling caused by foaming; and a large size for the evaporator, preventing the use of SAGD evaporators in mobile systems.
Hardness (mineral ions such as Ca2+ or Mg2+), scaling, and silica deposits can be controlled by limiting the concentration, increasing the pH of the water, or adding scale inhibitors such as calcium sulfate seed crystals. They can also be addressed by controlling the water recirculation through the falling film heat exchanger.
The majority of SAGD production facilities utilize hot or warm lime softening systems combined with Weak Acid Cation (WAC) ion exchange systems in order to treat produced and make-up water. However, this process does not produce a high quality boiler feed water and necessitates the use of Once Through Steam Generators (OTSG) which only partially boil the feed water (75-80%) in order to prevent scale deposition (by maintaining solids in solution in the unboiled water). This leads to energy inefficiency and excessive water disposal rates. Once through steam generators are custom built for the oil sands industry making them very costly compared to conventional boilers.
Recently, some SAGD operators have adopted falling film evaporators that produce a high quality distilled water for boiler feed water. This has made it possible to shift to more conventional drum boilers in the SAGD industry. The combination of falling film evaporators and drum boilers results in much higher water recycle rates (“WRR”) in an SAGD facility. This is becoming an increasingly critical environmental consideration.
However, operating companies are finding that there are many shortcomings with the current industry practice and evaporator system when employed in SAGD facilities. Improvements to the current state of falling film evaporator design for an SAGD water treatment system have focused on the five most problematic technical issues that have been observed in the field. These include:
1) preventing accumulation of hydrocarbons in the evaporator sump;
2) ensuring silica, calcium, and other water soluble contaminants are maintained in solution to prevent scaling on the evaporator heat transfer tubes;
3) selecting materials of construction suitable to the environment including high levels of chlorides in the evaporator sump due to the use of non-portable saline make-up water, pH levels in the sump, and/or the need to concentrate the brine to maximize water recycling;
4) minimizing power consumption in a water treatment unit, wherein all of the recovered water is evaporated and recondensed; and,
5) minimizing corrosion in the compressor and the suction piping to the compressor caused by the brine liquid carryover into the compressor.
One unique shortcoming not addressed by known current designs is the tendency of residual oil (including hydrocarbons, heavy oil, or SAGD emulsifiers/reverse emulsifiers) to accumulate in the evaporator sump. The typical designs withdraw a brine blowdown from the evaporator sump at the outlet of the evaporator recirculation pump. Owing to its lower density, oil will tend to slowly build up upon the surface of the water in the evaporator sump. To control accumulation of dissolved solids in the evaporator sump, a controlled volume of water is removed from the system at the discharge of the brine recirculation pump. However, oil that accumulates on the surface of the water in the evaporator sump cannot enter the brine recirculation pump since the pump suction line is drawn from the bottom of the evaporator sump. The accumulation of oil on the surface to the evaporator sump will lead to “foaming” events in the evaporator sump, fouling of heat exchange surfaces in the evaporator exchanger leading to a shutdown of the evaporator system in order to withdraw accumulated oil in the evaporator sump. The need to shut down the evaporator in order to deal with foaming events reduces the overall reliability of the SAGD plant and reduces the production volumes. It would thus be desirable to remove the oil that accumulates on the surface of the water in the evaporator sump on a continuous basis in order to prevent the foaming effect.
The operation of the evaporator is time and labor consuming and has to be highly controlled before, during, and after the operation. A typical control scheme for an evaporator includes the following factors:
1) The blowdown flow set point is changed by an operator in response to a lab analysis of the concentration of solutes (silica, chloride, etc.) in the evaporator sump so that the concentration of solutes is controlled manually.
2) The evaporator feed rate is adjusted automatically by a sump level controller in response to changes in sump level.
3) The compressor speed and or guide vane position is adjusted in response to the level of water in the distillate tank.
4) The production rate of distillate water from the evaporator is changed slowly in response to the level of the downstream tank. In extremes, the production rate is changed in response to the level of the feed tank.
5) Start up and shut down of the evaporator is done manually, causing significant time pressures on the operator. The mode changes, especially at startups, and the response time immediately after a trip or malfunction are the most dangerous times in a process plant.
6) Operators may make manual adjustments to rates to manage the inventories in 1) the upstream produced water tank which feeds the evaporator and 2) the downstream boiler feed water tank that holds the evaporator product distillate water.
In an attempt to deal with oil accumulation in the evaporator sump, conventional designs have tried to employ a skim draw at some specific level in the sump. However, this scheme can only be effective if the sump level is precisely controlled at a level just above the skim draw nozzle. If the level is too high above the draw point, oil will accumulate. If, on the other hand, the level falls below the draw nozzle, no liquid flow will be drawn off and again oil will accumulate. As mentioned, oil accumulated in the evaporator sump causes excessive foaming. Anti-foam chemicals are then added to the feed water but the addition may not be adequate to deal with excessive foam caused by oil accumulating in the sump. In addition, antifoam chemicals are typically light hydrocarbons themselves, which will also accumulate in the evaporator sump, and will eventually aggravate the hydrocarbon foaming issue rather than resolving it.
The known MVC evaporator has a vessel designed to separate the liquid brine and steam that is produced in the tube side of the evaporator exchanger. This can be in the evaporator sump or in a dedicated compressor knockout drum located between the evaporator sump and the compressor section. In either case, removal of the entrained brine can be increased through known design parameters utilizing proper vessel sizing, demist pads with water wash, and chevron mist eliminators. However, there is a residual amount of brine which will be carried through these devices. These droplets will be carried into the compressor whereas a result of the heat of compression in the compressor, the outlet steam from the compressor is super-heated. This ensures that in the compressor, the brine droplets are evaporated, depositing the dissolved minerals as a salt on the surfaces of the compressor, and resulting in corrosion problems in the compressor. There are some designs that recycle the super-heated compressed discharge back to the compressor section for the sole purpose of evaporating any liquid droplets in the feed to the compressor. This moves the vaporization of the brine droplets back from the compressor into the compressor suction piping, resulting in the deposition of salts in the compressor piping and causing corrosion of the suction piping. It would be desirable to reduce the amount of brine carried through these devices. It would also be desirable to reduce the amount of oil which is accumulated in the evaporator sump.
The implementation of existing technology involves a vertical exchanger bundle mounted on top of an evaporator sump. The sump provides both liquid inventory for the brine recirculation pumps and vapor space for liquid/vapor disengagement. Mist eliminators are typically installed in the vapor space in the annulus area around the falling film heat exchanger bundle, above which is located outlet piping to the compressor suction. As a result, the evaporator is very tall relative to other SAGD equipment and has a vessel diameter which is significantly larger than that of the falling film exchanger. These dimensional features restrict the equipment capacity that can be easily modularized and transported. High labor costs and low productivity, which are typically associated with SAGD operations, have driven owners to seek modular construction techniques to minimize site construction. This has created a need for new evaporator designs for use in the modular SAGD technology/market development addressing all of the above-mentioned deficiencies.